Optical filter devices used as windows and containing a fluid as the filter medium are well known. U.S. Pat. No. 192,843 (Sloan) disclosed a simple window structure capable of being filled with a tinted liquid. Other structures for vehicle and building windows into which a filtering liquid may be introduced or withdrawn through conduits, valving, pumping and other hydraulic means have been disclosed in U.S. Pat. Nos. 2,378,591 (Solis), 2,439,553 (Winn, aircraft windows), 3,174,398 (Brauner, automobile windows), 3,724,929 (Lacy, automobile windows), and 4,093,352 (Pisar). Other inventions have described additional applications of the fluid media light filtering devices and means for introducing and withdrawing the fluid; for examples, see U.S. Pat. Nos. 2,501,418 (Snowden, fluid flow activated by heating and cooling) and 3,674,344 (Lacy, motorcycle helmet and goggles, fluid flow by gravity). Some disclosures such as U.S. Pat. No. 3,724,929 (Lacy) have provided for the use of multiple immiscible liquids of differing specific gravities so that the filtering liquid may be selectively changed from one transmissivity to another. Still other less closely related inventions, of which U.S. Pat. No. 3,711,189 (Novotny) is an example, use the principle of internal reflection at a junction of materials of differing indices of refraction, to construct filters having selectably opaque and translucent states. More esoteric methods of darkening windows in the presence of solar radiation have been disclosed in French Pat. No. 1,584,280 (photochromic materials) and Belgian Pat. No. 677,236 (polarized photo-conductors).
Prior inventions, especially those using multiple immiscible filtering liquids, have suffered from a number of disadvantages. Those devices containing a conventional fluid pumping means suffer from interphase mixing brought about by the turbulent stirring action of a pump. Conventionally pumped system devices containing gaseous fluids also have the disadvantage of inducing gas bubbles in the fluid during pumping. In the art, it is preferable that the volume of the fluid media and, consequently, the filter cavity be kept relatively small. As a result, variations of media and filter temperature can have a dramatic effect on the cavity volume and the pressure of the fluid within it. For that reason, prior art inventions generally include an elaborate check valve-pressure relief valve system or an auxiliary reservoir to hold overflowing fluid, making those inventions relatively costly and complex.